Most complex biological and chemical targets require the application of complementary multidimensional analysis tools and methods to compensate for target and matrix interferences. Correct analysis and separation is important to obtain reliable quantitative and qualitative information about a target. In this regard, mass spectrometers have been used extensively as detectors for various separation methods. However, until recently most spectral methods provided fragmentation patterns that were too complicated for quick and efficient analysis. The introduction of atmospheric pressure ionization (API) and matrix assisted laser desorption ionization (MALDI) has improved results substantially. For instance, these methods provide significantly reduced fragmentation patterns and high sensitivity for analysis of a wide variety of volatile and non-volatile compounds. The techniques have also had success on a broad based level of compounds including peptides, proteins, carbohydrates, oligosaccharides, natural products, cationic drugs, organoarsenic compounds, cyclic glucans, taxol, taxol derivatives, metalloporphyrins, porphyrins, kerogens, cyclic siloxanes, aromatic polyester dendrimers, oligodeoxynucleotides, polyaromatic hydrocarbons, polymers and lipids.
According to the MALDI method of ionization, the analyte and matrix in solution is applied to a probe or target substrate. As the solvent evaporates, the analyte and matrix co-precipitate out of solution to form a solid solution of the analyte in the matrix on the target substrate. The co-precipitate is then irradiated with a short laser pulse inducing the accumulation of a large amount of energy in the co-precipitate through electronic excitation or molecular vibration of the matrix molecules. The matrix dissipates the energy by desorption, carrying along the analyte into the gaseous phase. During this desorption process, ions are formed by charge transfer between the photo-excited matrix and analyte.
Conventionally, the MALDI technique of ionization is performed using a time-off-flight analyzer, although other mass analyzers such as an ion trap, an ion cyclotron resonance mass spectrometer and quadrupole time-of-flight are also used. These analyzers, however, must operate under high vacuum, which among other things may limit the target throughput, reduce resolution, capture efficiency, and make testing targets more difficult and expensive to perform.
To overcome the above-mentioned disadvantages in MALDI, a technique referred to as AP-MALDI has been developed. This technique employs the MALDI technique of ionization, but at atmospheric pressure. The MALDI and the AP-MALDI ionization techniques have much in common. For instance, both techniques are based on the process of pulsed laser beam desorption/ionization of a solid-state target material resulting in production of gas phase analyte molecular ions.
AP-MALDI can provide detection of a molecular mass up to 106 Da from a target size in the attamole range. In addition, as large groups of proteins, peptides or other compounds are being processed and analyzed by these instruments, levels of sensitivity become increasingly important. Various structural and instrument changes have been made to MALDI mass spectrometers in an effort to improve sensitivity. Additions of parts and components, however, provides for increased instrument cost. In addition, attempts have been made to improve sensitivity by altering the analyte matrix mixed with the target. These additions and changes, however, have provided limited improvements in sensitivity with added cost. More recently, the qualitative and quantitative effects of heat on performance of AP-MALDI has been studied and assessed. In particular, it is believed that the performance of an unheated (room temperature) AP-MALDI source is quite poor due to the large and varying clusters produced in the analyte ions. These large clusters are formed and stabilized by collisions at atmospheric pressure. The results of different AP-MALDI matrixes to different levels of heat have been studied. In particular, studies have focused on heating the transfer capillary near the source. These studies show some limited improvement in overall instrument sensitivity. A drawback of this technique is that heating and thermal conductivity of the system is limited by the materials used in the capillary. Furthermore, sensitivity of the AP MALDI source has been limited by a number of factors including the geometry of the target as well as its position relative to the capillary, the laser beam energy density on the target surface, and the general flow dynamics of the system. Thus, there is a need to improve the sensitivity and results of AP-MALDI mass spectrometers for increased and efficient ion enhancement.